Method and device for determining the speed of a member

ABSTRACT

A method and apparatus for controlling the rotational speed of a roller. The actual speed of the roller which is compared with a set speed in order to determine the deviation or error, is derived by means of a pulse disc which bears a plurality of pulse producing fields. The time interval which is required for each pulse flank to pass successively beyond two detectors is measured, and the inverse value of said interval instantly indicates the actual roller speed so that at each angular position of the roller, the roller speed may be exactly controlled.

United States Patent 1 Natens [451 Dec. 4, 1973 METHOD AND DEVICE FOR DETERMINING THE SPEED OF A MEMBER [75] Inventor:

[73] Assignee: Agfa-Gevaert N.V., Mortsel,

Belgium 22 Filed: Oct. 22, 1971 21 App]. No.: 191,850

Luc Yves Natens, Bel-chem, Belgium [30] Foreign Application Priority Data Oct. 23, 1970 Great Britain 50,498/70 [52] US. Cl 318/313, 318/327, 324/175 [51] Int. Cl. H02p 5/16.

[58] Field of Search 318/326, 327, 328, 318/313, 314, 318;324/175,178

[56] References Cited UNITED STATES PATENTS 4/1969 Lundgreen 324/175 8/1972 Schwartz.....

3,586,949 6/1971 Spear; 318/326 2,794,945 6/1957 Lelmer... 318/313 3,581,176 5/1971 Bigg 318/327 3,582,699 6/1971 Badessa 318/327 Primary Examiner-Bemard A. Gilheany Assistant ExaminerThomas Langer Attorney-William J. Daniel [5 7] ABSTRACT A method and apparatus for controlling the rotational speed of a roller. The actual speed of the roller which is compared with a set speed in order to determine the deviation or error, is derived by means of a pulse disc which bears a plurality of pulse producing fields. The time interval which is required for each pulse flank to pass successively beyond two detectors is measured, and the inverse value of said interval instantly indicates the actual roller speed so that at each angular position of the roller, the roller speed may be exactly controlled.

6 Claims, 10 Drawing Figures PATENTEDDEC 41975 SHEET 1 OF 3 Fig. I

'1 METHOD AND DEVICE FOR DETERMINING THE SPEED OF A MEMBER The present invention relates to a method and apparatus for controlling the rotational speed of a roller, in particular of a web supporting roller for conveying a web past a coating'system.

In the coating of a web the coating composition is applied by dipping, extrusion, cascade or the like to one face of a continuously travelling web while the web is being supported at the rear side by a roller, called hereinafter backing roller. The rate of wet coating composition applied per surface unit of the web depends on the speed of the web, and this in particular when the coating composition is being stretched in a coating arrangement wherein the speed of the web is higher than the speed of supply of the composition.

In the manufacture of photographic light-sensitive material the control of the speed of the web at the position of the coating station is of paramount importance and for that reason thespeed of a web in modern photographic coating devices is no longer controlled by driven rollers disposed before and beyond the backing roller but, on the contrary, the backing roller itself is driven. The driving of the backing roller occurs by a motor which is directly mounted on a shaft extremity of the roller, and which motor has a high torque and a low inertia, such as a printed motor or a so-called pancake motor. Changes in the speed of a web which may be due to fluttering of the web in the drying chamber, the splicing of a fresh web to the receding web, etc., may be almost instantly corrected by such driving system.

Known devices for measuring the rotational speed of a roller do not permit to obtain the full benefits of the described driving system because the response of these devices is too slow. They usually comprise a disc which is provided at the circumference with equally spaced light-absorbing and light-transmitting areas and which may be mounted at the extremity of the shaft of the roller to be measured. A photohead comprising an incandescent light source and a photoelectric cell is placed over the disc so that the successive areas of the disc generate pulses, the frequency of which is a measure of the speed to be determined. This device has the follow ing disadvantages.

The pulse-producing elements of the disc must have exactly the same characteristics because any lack of correspondence between these elements may affect the registered pulse frequency so that this is not a measure of the speed to be determined.

Errors are liable to arise due to parallax or to an inaccurate placing of the photohead relative to the disc, and therefore the free distance between the disc and the photocell, and between the disc and the lamp does normally not exceed a few tenths of a millimeter.

The pulse train frequency has to be converted into an analogous value which is an indication of the speed of the roller. Actually, this conversion constitutes the most important drawback of the known devices since the period of the measurement signal becomes too great. For instance, a disc rotating at one revolution per second and being provided with 1,000 opaque strips on a transparent base, generates a frequency of 1,000 Hertz, what means a pulse period of l msec. Since the measurement signal contains a 100 percent ripple, it must be electronically smoothed by an RC-network in order to contain finally only, say one percent of ripple. This ripple-level is obtained after only about 50 pulses, so the time constant 1 of the system becomes 328 msec T= 21rf= 6.28 x 50).

It is possible to obtain short time constants by operating according to the so-called phase-lock system. However, in this system still a high precision is required for the pulse producing member and, in addition, a very complicated electronic circuitry is necessary since the phase measurement must be coupled'with a frequency measurement, and as a consequence thereof an electronic arrangement with very particular transfer functions is obtained. 7

The present invention aims to provide a method for controlling the rotational speed of a roller, which permits to obtain a very quick response and thereby enables an optimal profit of the advantages of modern high torque driving motors.

The invention aims further to provide an improved control apparatus, the pulse producing member of which is not liable to close tolerances and the mounting of which is not critical.

Finally the invention aims to provide a control apparatus which does not require a sophisticated electronic circuitry.

According to the present invention, the method for controlling the rotational speed of a roller comprises measuring the rotational speed of said roller, compar ing the measurement value with a setspeed value, feeding the deviation between both signals to a controller which controls a motor driving said roller, the measuring of the rotational speed of said roller occurring by means of a member which moves in synchronism with said roller and which is provided with a plurality of pulse producing elements, two detectors being provided and being located at fixed positions such that they are acted on successively to cause successive signal pulses to be produced by the movement of said pulse producing elements, and the interval of time between the successive signal pulses being used in the comparison with the set speed value.

The term roller" should be construed sufficiently broad to include also other rotatable members such as a wheel rotatable around a horizontal axis and which is capable of supporting a webon an angular portion of its periphery, a disc or turntable rotatable around a vertical axis and which is capable of supporting a load on the upper face, etc.

The apparatus for controlling the rotational speed of a roller driven by a motor according to the present invention comprises a controller controlling the electric current of the motor driving said roller, a comparator for comparing a set speed value with a measured speed value and for feeding the difference between both said values to the said controller, and means for determining the measured speed value, said latter means comprising a member having a plurality of pulse producing elements, an electromagnetic radiation source for directing an amount of energy towards the member, two

on the member and the moment said point is detected by the second detector.

Preferred embodiments of the apparatus according to the invention are as follows.

The apparatus comprises means for determining the inverse value of the said time interval.

The member having a number of pulse producing elements is in the form of a transparent disc which is provided near its periphery with a plurality of radiation absorbing fields.

The two detectors are mounted at equal distances from the center of rotation around which the disc rotates. Thereby the real peripheral speed of a pulse flank of a pulse disc is measured along the periphery of a circle thecenter of which coincides with the rotation of the pulse disc. The advantage of such measurement lies in its independence of the centering of the pulse disc.

The invention will be described more in detail by way of an example with reference to the accompanying drawings, wherein:

FIG. 1 is a block diagram of the control apparatus according to the invention.

FIG. 2 is a diagrammatic view of a coating station.

FIG. 3 is a plan view of the means for determining the rotational speed of a roller.

FIG. 4 is a front view of the pulse disc of the arrangement according to FIG. 3.

FIG. 5 is another embodiment of the optical arrangement according to FIG. 3, and

FIG. 6 is still another embodiment of this optical arrangement.

FIG. 7 represents three electric signals elucidating the method according to the invention.

FIG. 8 is a graph showing the relation between a variable and its inverse value.

FIG. 9 is an electronic circuit for determining a time interval, and

FIG. 10 is an electronic circuit for determining the inverse value of the output signal of the circuit of FIG. 9.

In the block diagram according to FIG. 1, a rotatable roller 70 is driven by a high torque motor 71. The rotational speed of the roller 70 is measured by means of a pulse disc 72, a photohead 73 and an electronic circuit 74 which determines the inverse value of the time interval between successive pulses produced by themovement of a pulse flank of the disc in front of two detectors in the photohead 73. The inverse value constitutes the measured speed value v,,, and is compared in a comparator circuit 75 with a set speed value v, which is set in a circuit 76. The deviation or error signal determined by the comparator 75 is fed to a controller 77 which controls the current of the motor 71 in order to obtain the desired rotational speed of the roller 70.

The roller 70 may be the backing roller in a dip coating system as is diagrammatically shown in FIG. 2 wherein a web 78 to be coated is pulled over an idle roller 79 and passed around a substantial angular portion of the backing roller 70. The web dips in the coating composition in a tray 80 and is pulled over a further roller 81 to a drying station, a second coating station, etc. Means known in the art control the tension of the web so that the frictional contact between the rear side of the web and the roller 70 is sufficiently great to enable the roller 70 to control the speed of the web.

The arrangement of the pulse disc and the photohead is shown more detailed in FIG. 3. The shaft 10 of the 4 roller (the driving motor has been deleted in the figure) bears the pulse disc 72 which is provided with alternating optically transparent and opaque fields near its periphery. At one side of the pulse disc two photo electric cells 12 and 13 are arranged in a holder 15 fitted in the photohead 73. g

Between the pulse disc 72 and the photoelectric cells 12 and 13 there is a diaphragm 14 having two slots at the locus of the photoelectric cells. It is important for the photoelectric cells 12 and 13 to be positioned at exactly the same distance from the axis of rotation of the pulse disc 72. This does not mean that the center of the disc must coincide with the axis of rotation thereof, and as a consequence thereof the disc need not be accurately centered in respect of shaft 10.

At the other side of the pulse disc 72 are mounted a light source 16 with the optical lens system 17 which directs an almost parallel light beam onto the disc at the locus of the photoelectric cells 12 and 13.

The numeral 18 in the arrangement of FIG. 3 refers to the light spot projected by the optical system 17, and is more clearly illustrated in FIG. 4.

As photoelectric cells use may be made of photoelectric transistors, e.g., of the type LS-400 of Texas Instruments or MRD 200 of Motorola (registered trademarks). These photoelectric transistors have a diameter of about 2.5 mm. This diameter limits the minimal pulse distance on the disc. The latter distance may, however, be reduced if for the reading out use is made of an optical system 69 which. projects an enlarged image of the flanks on the photoelectric cells, (see FIG. 5). This optical system must give a very true reproduction since it will cause measuring faults otherwise. An other reading out technique comprises the use of light conducting elements, e.g., fibre optics 68 and 67 which transmit the image to photoelectric cells, e.g., photoelectric multiplier tubes 40 and 41, see FIG. 6.

The pulse producing elements or windows can be arranged, e.g., on a glass disc with stability in shape and of high impact strength, on which glass disc a photographic emulsion of steep gradation is coated as a thin layer having a thickness in the order of magnitude of 10 um. The windows are formed on the disc, e.g., by producing successive slot-like flash exposures thereon, while said disc is rotatably fitted on the distributor head of a milling machine.

Due to the insensibility of the measuring system with respect to an irregular pulse succession over the periphery, the accuracy thereby attained will usually suffice.

Instead of photographic, also mechanical .pulse discs may be manufactured, i.e., pulse discs having mechanically interrupted wall portions. The quality of the latter discs is, however, mostly'insufficient due to an inadequately abrupt transition from signalling -to nonsignalling condition. There is usually a diffuse dispersion of light on the relatively thick crenelated walls. In comparison thereto, the small layer thickness of photographic materials offers an important advantage in permitting very steep pulse flanks.

The measurement of the time elapsing' between the detection of one and the same flank by both photoelectric cells is done electronically by feeding an integratormemory so that one disposes of an accurate value of the period at the end of the measuring time.

In FIG. 7 the pulse current 30, the integrator voltage 31 and the voltage 32 recorded in a memory, are illustrated.

In most cases, the value of the angular speed of the disc or other member to be tested for speed is of predominant importance. This angular speed is inversely proportional to the pulse period. Several methods are known for determining electrically the reciprocal value 10 of a voltage. If only a narrow speed range is envisaged then use can be made of the tangent, at some defined point, to the hyperbola giving the relation between a variable E and its reciprocal value E l/E,. FIG. 8

represents a graph indicating the relation E versus E If a greater speed range is required appeal may be made to non-linear techniques for converting the hyperbola according to FIG. 8 to a straight line, e.g., by using diode arrays the attenuation of which is inversely proportional tothe applied voltage. This method may be made to work very accurately but requires a great deal of adjusting activity. For the latter reason it is mostly preferred to make a real division.

In FIG. 9 an electric circuit is shown for measuring the pulse period. The output signals of the photocells 12 and 13 are connected to the triggers 50 and 51, respectively. The output signals of the triggers control a flip-flop 52, itself controlling an integrator 53. At the ending of the measuring period a sample and hold circuit 54 is started through a monostable multivibrator 57 so that a new measuring cycle may start.

The output signal E of the circuit according to FIG. 9 is directly proportional to the period. So, the inverse values thereof must be determined. For this purpose an electronic divider circuit is used which is based on the principle of time division by means of modulation of the pulse width.

Be E, the output voltage of the circuit according to FIG. 9 the inverse value of which must be determined.

The width of the pulses of a pulse train e, with a sufficiently high frequency is modulated with B, so that E, ka wherein a is the one/off ratio of the pulse train e,.

By means of the pulse train 2, the feedback current of an operational amplifier 60 represented in FIG. 10

is modulated. A capacitor C is connected .between the input and the output of the amplifier 60, and a resistor R may be also connected over an amplifier by means of a switch 61 which is controlled by the pulse train e The output voltage of the amplifier 60 is E; R f'i wherein i is the input current. of the amplifier and R is the effective feedback resistance. But E R i wherein i' is the value of the current through R when the switch 61 is closed, and wherein i'=i since no charge can remain accumulated in the capacitor C at steady condition. Further i' on, so E R' i R/a 1 5 If i is kept to a constant value then the circuit of FIG. 10 acts as a divider, the signal E corresponding to the signal v,, according to FIG. 1. The output ripple is determined by the modulation frequency and the time' 0 In an arrangement according to the present inven- 5 tion, the diameter of a glass pulse disc amounted to 50 cm, the thickness of the disc amounted to 3 mm, and the periphery of the disc was provided with 600 small black fields. The clearance between the disc and the lamp, and between the disc and the photocell of the photohead amounted to 7 mm. It has been shown that the disc could be mounted out of center to 1 mm, and that the wandering of the disc, i.e., its mounting out of the plane normal to the shaft, could amount to 2 mm, and that yet an accuracy of control of one per 1,000 could be-obtained for revolutions ranging from 0.1 to 10 per minute.

We claim:

1. In an apparatus for controlling the speed of a motor including means for observing the actual speed of said motor and for providing a signalcorresponding to such observed motor speed, means for providing a reference speed signal, a comparator receiving said actual and reference speed signals for determining the different therebetween and providing an error signal corresponding to such difference, and a controller receiving said error signal from said comparator and controlling the speed of said motor accordingly, an improved observed speed measuring means comprising a pulse producing member rotated by said motor, said member having a plurality of radiation-intercepting elements arranged in spaced relation thereon, an electromagnetic radiation source disposed on the path of said radiation-intercepting elements and impinging radiation on said intercepting elements in succession to pro duce by the intermittent interception of said radiation by said elements a series of radiation pulses, two radiation-sensitive detectors arranged in spaced relation along the path of said intercepting elements to receive said radiation pulses, said detectors being arranged such that both receive the pulse corresponding to the same intercepting element before receiving the pulse corresponding to any other element, and means for determining the interval of time elapsing between the instant said pulse is first detected by said firsi detector and the instant the same pulse is first detected by said second detector, said time interval being indicative to the speed of said pulse-producing member and thus of said motor.

2. Apparatus according to claim 1, wherein said pulse-producing member is in the form of a transparent disc, which is provided near its periphery with a plurality of radiation absorbing sectors.

3. Apparatus according to claim 2, wherein the two detectors are mounted concentrically with the center of rotation of said disc.

4. Apparatus according to claim 2, wherein the electromagnetic energy source is a light source and wherein between the disc-and the detectors an enlarging optical system is mounted.

5. Apparatus according to claim 2, wherein light conductors are mounted with their ends proximate the path of said intercepting elements for transmitting the radiation pulses to the respective detectors.

6. Apparatus according to claim 1, comprising means for determining an inverse value of said time interval, said inverse value being directly proportional to the angular speed of rotation of said member.

I l i 

1. In an apparatus for controlling the speed of a motor including means for observing the actual speed of said motor and for providing a signal corresponding to such observed motor speed, means for providing a reference speed signal, a comparator receiving said actual and reference speed signals for determining the different therebetween and providing an error signal corresponding to such difference, and a controller receiving said error signal from said comparator and controlling the speed of said motor accordingly, an improved observed speed measuring means comprising a pulse producing member rotated by said motor, said member having a plurality of radiation-intercepting elements arranged in spaced relation thereon, an electromagnetic radiation source disposed on the path of said radiation-intercepting eLements and impinging radiation on said intercepting elements in succession to produce by the intermittent interception of said radiation by said elements a series of radiation pulses, two radiation-sensitive detectors arranged in spaced relation along the path of said intercepting elements to receive said radiation pulses, said detectors being arranged such that both receive the pulse corresponding to the same intercepting element before receiving the pulse corresponding to any other element, and means for determining the interval of time elapsing between the instant said pulse is first detected by said first detector and the instant the same pulse is first detected by said second detector, said time interval being indicative to the speed of said pulseproducing member and thus of said motor.
 2. Apparatus according to claim 1, wherein said pulse-producing member is in the form of a transparent disc, which is provided near its periphery with a plurality of radiation absorbing sectors.
 3. Apparatus according to claim 2, wherein the two detectors are mounted concentrically with the center of rotation of said disc.
 4. Apparatus according to claim 2, wherein the electromagnetic energy source is a light source and wherein between the disc and the detectors an enlarging optical system is mounted.
 5. Apparatus according to claim 2, wherein light conductors are mounted with their ends proximate the path of said intercepting elements for transmitting the radiation pulses to the respective detectors.
 6. Apparatus according to claim 1, comprising means for determining an inverse value of said time interval, said inverse value being directly proportional to the angular speed of rotation of said member. 